1. Field of the Invention
The present invention relates to a multiplexer in a data system. More particularly, the present invention relates to an analog amplifier-multiplexer utilized for a read channel in a data system.
2. Related Art
In a data processing system, it is sometimes necessary to multiplex a plurality of data lines onto a single data transmission line. For example, FIG. 1(a) illustrates a portion of a data system in which such multiplexing is necessary. In FIG. 1(a), there are a plurality of read channel lines 11. Each read channel line 11 is connected to a transducer head (not shown) for reading data from a recording medium. Such a recording medium may be a magnetic disk, magnetic tape, optical disk or other such media. A plurality of these read channel lines 11 are inputs to a pre-amplifier 12. In the example shown in FIG. 1(a) there are 13 read channel lines however, a greater or lesser number of read channels may be utilized. If for example each pre-amplifier 12 can interface with four read channel lines 11 as inputs then a minimum of four pre-amplifiers 12 are necessary for the exemplary system. Each pre-amplifier 12 has as an output a pair of differential signal lines 14. It is necessary that the four pairs of differential signal lines from the pre-amplifiers 12 of FIG. 1(a) be reduced to a single pair of differential signal lines for purposes of inputting the data signals appearing on all of those differential signal lines into a portion of the system that constitutes additional read circuitry of the data system.
One possible solution would be to tie together the four pairs of differential lines 14 so as to produce one pair of differential lines. This alternative has many disadvantages, one of the disadvantages being an introduction of different levels of noise due to the necessity of each pair of differential lines being of a different length so that they can all be joined together. In addition, if this solution is used there is no amplification provided to the data signals on the lines and there is no true multiplexing of the data signals for creating an input to the additional read circuitry.
A second solution is to provide a plurality of FET switches which are turned on and off by a logic signal. This set of switches acts as an analog multiplexer and can multiplex four pairs of differential signal lines down to one pair of differential signal lines. A commercially available device, the Harris HI-509L consists of a simple pair of FET switches that are turned on and off by a logic signal. Available units have no specification for bandwidth, or common mode rejection and there is no gain. It is believed that none of the commercial parts have high bandwidth, high common mode rejection and gain in a single package.
Therefore, previously available means for multiplexing a plurality of pairs of differential signal lines down to a single pair of differential signal lines do not have characteristics which are desirable for providing an amplified-multiplexed signal to additional read circuitry. Known multiplexers used in this environment do not have suitable gain capabilities, do not have high bandwidth and do not have high common mode rejection characteristics.